A bridge for roads such as viaducts of expressways (hereinafter, referred to as road bridge) is constituted, as a main structure, of bridge piers fixed on the ground, a main girder stretched between the bridge piers, a deck placed on the main girder, and pavement laid on the deck. Decks for road bridges, having a function of transferring a load of a vehicle running on the pavement to the bridge piers through the main girder, have been developed in various types, including RC decks formed from reinforced concrete, steel floor systems formed from steel material, composite decks formed from reinforced concrete and steel material, and the like.
To a deck of a road bridge, there may occur damage such as cracks and fissures due to repeated loads by running vehicles on the pavement. With such damage furthered, vicinities of damage are repeatedly displaced with load actions, causing the damage to be escalated with the result of occurrence of potholes, peeling or other damage at portions of the pavement located above the damage portion. Thus, damage to the deck causes damage to the pavement and moreover adversely affects traffic of vehicles, making it desirable to detect any damage to the deck in early stages. However, since the deck is covered with the pavement, it is difficult to detect any damage to the deck from the top surface side of the pavement. In many cases, detection of any damage to the deck is not achieved until damaged pavement is removed for repairing due to occurrence of potholes or the like in the pavement.
Conventionally proposed as a method for detecting damage to the deck of the road bridge in early stages is one using thermal videos of thermography (see Patent Literature 1). In this method for detecting damage to a deck, a pavement material of about 160° C. temperatures is laid on an upper side face of an RC deck, and temperature distributions shown on a lower side face derived from conduction of heat of the pavement material through the RC deck are acquired by a thermal infrared video camera from downward of the RC deck. With presence of cavities or voids as defective portions in the RC deck, temperature conduction is inhibited, so that surface temperatures of sites lower than the defective portions are shown as low-temperature regions as compared with sound portions. Accordingly, it is considered that presence of defective portions in the deck can be detected by extracting low-temperature regions in the lower side face of the deck.    Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2003-247964